1. Field of the Invention
The present invention relates to a method of forming a hydrophobic surface and more particularly, to a method of forming such a surface by exposure of the surface to a source of ultraviolet radiation.
2. Discussion of the Prior Art
For purposes of the instant disclosure, the terms "wettable" and "hydrophilic" are used synonymously and interchangeably with each other, as are the opposite terms "non-wettable" and "hydrophobic." Moreover, hydrophilicity is indicated by the existence of a condition termed "practical wetting."
"Practical wetting" is defined as the ability of a surface to retain, on a substantially macroscopically smooth, unroughened portion thereof, a continuous, thin, uniform layer of a liquid, such as water or other liquid medium, when the surface is held vertically, or in any other orientation. It should be noted that this definition does not necessarily refer to, nor depend on, contact angle, surface energy or surface tension. However, when practical wetting occurs, the contact angle goes to or closely approaches 0.degree.. Thus, the term "practical wetting" encompasses all types of wetting, due to whatever cause or causes, that meet the above definition of the term.
Generally speaking, whether a surface is wet or not depends to a great extent on the surface (often termed free-surface or interfacial) energies or tensions of (1) the surface to be wet, (2) the liquid with which the surface is to be wet, (3) the surface-liquid interface, and (4) the surface-vapor and liquid-vapor interfaces.
Thus, water, which has a relatively high surface energy or tension, is unable to wet a polytetrafluoroethylene (Teflon -- a T.M. of DuPont) surface or a polyethylene surface, both of which have very low surface energies or critical surface tensions. Water, however, will normally wet clean inorganic glasses which have a rather high surface energy or critical surface tension. Surface energies and tensions are extremely difficult to measure in many instances, and are often only qualitatively describable.
That the relative surface energies or critical surface tensions of a surface and a liquid are favorable to the occurrence of wetting has been traditionally indicated by, among other things, the character of the so-called contact angle and the presence of "spreading." The contact angle is that angle measured through a liquid droplet between (a) a flat surface on which the liquid droplet resides and (b) a line tangent to the envelope of the droplet where the envelope intersects the surface. However, there is some controversy as to the upper limits of contact angle size beyond which a condition of non-wetting is present. This controversy is one reason for the use herein of the term "practical wetting," defined earlier.
An indicia of the non-wetting of a surface by a liquid occurs when the surface is immersed in a liquid and upon removal therefrom the liquid draws back (the liquid does not wet the surface). Specifically, if a drop of a liquid "beads up" and assumes a spherical or nearly spherical shape, the liquid does not wet the surface. On the other hand, if the droplet on the surface spreads out over the surface, the liquid wets the surface. It has usually been observed that where spreading occurs, EQU .theta. = 0.degree.
at the surface-liquid interface. Obviously, this test is qualitative, and also presents difficulties where spreading is observed but the contact angle is greater than 0.degree..
The above definition and use of the term "practical wetting" eliminates differentiation of contact angle situations falling between the two extremes of 0.degree. and 180.degree., or between situations such as that of water on Teflon or on clean glass. Moreover, the difficulties presented by the concurrence of spreading and of a contact angle greater than 0.degree. are avoided. Instead, if a liquid is retained, in accordance with the above definition, practical wetting is present, contact angle, surface energy and other considerations aside. Conversely, if a liquid is not retained in accordance with the above definition, non-wetting occurs.
Hydrophilic materials such as glass, glazed or enameled articles, etc., are many times required to be hydrophobic, i.e., water repellent. For example, a common method of producing lithographic printing masters is by rendering selected portions either hydrophilic or hydrophobic (depending on the nature of a surface) whereby either an oil-based ink (non-aqueous) or a water-based ink is retained on the surface in a pattern. Such a technique is also employed to apply a conductive ink or an ink catalytic to electroless metal deposition in a pattern for the ultimate deposition of an electroless and/or an electroplated metal deposit thereon. Also it is customary to treat glass fibres with oils, waxes, etc., during manufacture in order to protect the glass surface from the effect of moisture and to prevent loss of tensile strength by abrasion. Another illustration of the deleterious effects of moisture is found in glass bodies for electrical insulating purposes which, when exposed to the weather, lose their high electrical surface resistance under wet conditions. This is particularly true in the case of glass fibres having an extremely large surface per unit volume. A material which would help to preserve the high electrical surface resistance is highly desirable. Many other illustrations of the harmful effects of moisture exist but are too well known to require further discussion here.
Primary objects of this invention are to (1) render normally non-water-repellent, i.e., wettable, base members water-repellent, i.e., non-wettable and (2) selectively render a surface (originally hydrophobic or hydrophilic) hydrophobic to obtain a hydrophobic pattern.